A polyarylene sulfide (also referred to as “PAS” hereinafter), typified by polyphenylene sulfide (also referred to as “PPS” hereinafter), is an engineering plastic having excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like. Polyarylene sulfides are frequently used in a wide range of fields such as electrical/electronic equipment and automobile equipment due to their moldability into various molded products, films, sheets, fibers, and the like by general melt processing methods such as extrusion molding, injection molding, and compression molding.
A known typical method of producing a PAS is to polymerize a dihalo aromatic compound such as p-dichlorobenzene (also referred to as “PDCB” hereinafter) and a sulfur source containing an alkali metal, as an aqueous mixture containing a polar organic solvent such as N-methyl-2-pyrrolidone (also referred to as “NMP” hereinafter) while heating (for example, under temperature conditions around 175 to 350° C.) (Patent Documents 1 and 2), thereby obtaining a PAS such as PPS. Another known method to produce a high-molecular-weight PAS is a two-stage polymerization method whereby polymerization is performed while varying the polymerization temperature and the water content present in the polymerization reaction system (Patent Documents 3 and 4).
According to the PAS production method in which a dihalo aromatic compound and a sulfur source containing an alkali metal are polymerized as an aqueous mixture containing a polar organic solvent while heating, a PAS is produced via the reaction below. Specifically, when PDCB (denoted as “Cl-φ-Cl”) is used as the dihalo aromatic compound, sodium sulfide (Na2S) is used as the sulfur source containing an alkali metal, and NMP is used as the polar organic solvent, PPS {—(-φ-S—)n—} is produced according to the following Formula 1:nCl-φ-Cl+nNa2S→—(-φ-S—)n—+2nNaCl  (Formula 1)and at the same time, a large amount of NaCl, namely 2n times the molar quantity of the obtained PPS, is produced as a by-product. For this reason, it is difficult to increase the production quantity per polymerizer (polymerization vessel). Furthermore, since the by-product NaCl does not dissolve in NMP, it is present as a solid in an ordinary polymerization reaction system, and therefore it attaches to and contaminates the walls of the polymerizer (e.g., polymerization vessel) and causes abrasion of those walls. Since NaCl must be removed by washing, the burden of post-treatment is also large.
Other known typical methods for producing a PAS include a method in which PPS (a polythioether) is obtained by condensing diphenylsulfide and sulfur chloride in the presence of at least one substance selected from the group consisting of iron, aluminum, aluminum amalgam, Lewis acids, and protonic acids (Patent Document 5); a method of producing PPS in which 4,4′-dimercapto diphenylsulfide and 4,4′-dihalo diphenylsulfide are polymerized in a solvent while being held at a temperature of 200 to 400° C. in the presence of an alkali carbonate or an alkali bicarbonate (Patent Document 6); and a method in which a thiophenol and a halogenated aromatic compound are polymerized. However, the sulfur sources differ in these PAS production methods, and the polymerization reactions proceed via completely different reaction mechanisms than the reaction mechanism described above.
In the PAS production method wherein a dihalo aromatic compound and a sulfur source containing an alkali metal are polymerized as an aqueous mixture containing a polar organic solvent while heating, the polymerization temperature must be raised as high as possible in order to increase the polymerization rate, but conditions must be strictly selected because it will be accompanied by polymerization reactions in the polar organic solvent occurring at higher temperatures. For example, the proportions of dihalo aromatic compound, sulfur source containing alkali metal, and polar organic solvent are optimized, and the like. When the polymerization temperature is increased, various side reactions may readily occur. For example, the side reaction product produced when the polar organic solvent NMP is ring-opened and the open-ring NMP reacts with the dihalo aromatic compound PDCB (chlorophenyl methyl aminobutyric acid (also referred to as “CPMABA” hereinafter)) bonds with the molecular terminals of the PAS to inhibit the polymerization reaction, and there is the risk that a high-molecular-weight PAS can no longer be obtained or that yield will decrease due to consumption of the dihalo aromatic compound (PDCB) which is a PAS starting material.
The polymerization reaction between the dihalo aromatic compound and the sulfur source containing an alkali metal is affected by the water content present in the polymerization reaction system. In particular, when the polymerization reaction is performed at high temperature in order to obtain a high reaction rate, abnormal reactions such as a decomposition reaction or the like may readily occur if a large amount of water and a sulfur source containing an alkali metal are present in the polymerization reaction system. Therefore, a dehydration step is often required prior to the step of preparing (also referred to as the “charging step” hereinafter) the mixture (also referred to as the “charged mixture” hereinafter) of dihalo aromatic compound, sulfur source containing an alkali metal, polar organic solvent, and water to be submitted to the polymerization reaction. In the dehydration step, unnecessary water is expelled to outside the system under normal pressure while the sulfur source containing an alkali metal and the polar organic solvent are heated to a temperature of, for example, approximately 150 to 210° C. Because sulfur sources containing an alkali metal, e.g., sodium sulfide (Na2S), often contain water of crystallization, it is normally necessary to reduce the water of crystallization. In many cases, the required dehydration step is performed using the apparatus that performs the polymerization reaction, and there are also cases where it constitutes from 20 to 50% of the polymerization time. Thus, this is one of the problems to be solved to increase PAS productivity.
Since PASs such as PPS, which are engineering plastics having excellent heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical characteristics, dimensional stability, and the like, have come to be used in a wide range of fields, a more efficient method of producing PASs has come to be demanded. Specifically, in a method of producing a PAS in which a dihalo aromatic compound and a sulfur source containing an alkali metal are polymerized in an organic amide solvent which is a polar organic solvent, a method of producing a PAS whereby production of reaction by-products such as CPMABA is suppressed and PAS with excellent practical utility is produced at a higher yield, a polymerization method whereby the amount of NaCl produced as a by-product of PAS production is reduced, and a method whereby, due to a dehydration step being omissible, PAS with excellent practical utility is produced with high productivity in a short time, are also demanded.